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The motor soft starter is an electronic motor starting device. It is intended to provide three-phase induction motors with a smooth start, both mechanically and electrically. Soft starters utilize thyristors in a full-wave power bridge confi guration. By varying the thyristor conduction period, it controls the voltage applied to the motor. Thus it controls the torque developed by the motor. After the motor reaches its designed speed, contactors are closed to bypass the thyristors. 

The SSRV starter uses high speed switching devices (SCRs or Silicon Controlled Rectifiers) to switch on for only a portion of each half of the sine-wave line power. By doing so, the RMS (roughly average) voltage getting to the motor is reduced proportionately by the amount of time the switch is delayed. So if the Silicon Controlled Rectifier (SCR) is not allowed to begin conducting (know as being "gated") until the sine-wave is already 1/2 over with, the output RMS voltage will be 1/2 of the line voltage. By moving the "gate" point further back in the sine-wave, the RMS voltage is increased until the Silicon Controlled Rectifier (SCR) is being gated at the Zero-cross point and the motor is getting full line voltage.  

Similar to Reduced Voltage Auto Transformer (RVAT) starting are Reactor and Primary Resistor starters, which drop the voltage through those devices as well.  

Electrically motor soft starter can be any devices that reduces the torque through reducing voltage, or a change in the motor connection.  

With this traditional way, iff the load became jammed, the sudden rise in torque would cause the pin to shear so that the two shafts could then rotate independently, thereby disconnecting the motor from its load. Before the load could be restarted, the old pin would need to be removed and a new one inserted - an obviously inconvenient and time-consuming process.

The electronic shear pin facility eliminates the need for a mechanical shear pin entirely because the speed and extent of a sudden and rapid rise in motor torque is immediately detected which will then decide on a course of actions ranging from instantaneous shutdown to monitoring for recurrences if the blockage is released rapidly.

Soft starters provide an electronic solution to mechanical problems at relatively low cost. They can extend the life of belts, chains, gearboxes, shafts, bearings and machine mountings.

Similar savings can also result if there is a breakdown situation. Let's take the example of a blockage in a mill. Historically, all other motors have been left running when this situation occurred, even though the time required to remove the blockage was considerable. But today, this cost can be avoided simply by employing soft starters on the motors, enabling them to be switched off with no negative consequences for restarting. The example above deals with a breakdown situation.

But prevention is always better than cure. Modern manufacturing processes employ large numbers of pumps to convey everything from water to hazardous fluids. In many applications, these pumps are driven by motors. These motors have no form of control during their starting and stopping.

Sudden torque stresses will cause excessive wear on belts, pulleys, gears, chains, couplings and bearings, and also cavitation in pumps, which reduces the efficiency and life ot those equipments.

Similarly, shock waves can be generated and transmitted along hydraulic pipework, weakening joints in pumping systems. And in conveyor systems, loads may be displaced or damaged on startup, and products may become contaminated.

Clearly then, mechanical engineers have more reasons to press for the fitting of soft starts in fixed speed motor applications than electrical engineers. Especially so, as the cost savings resulting from reduced downtime and from not having to replace bearings, gears, pulleys and bearings so frequently, will ensure quick payback from investment on any soft starter unit.

The electrical engineer benefits in two ways as below:

1. by avoiding the dips in mains voltages that occur due to current peaks inherent in "Across the Line Starting". 
2. by avoiding the considerable stresses on the motor windings, and the iron cores of the stator and rotor, which will result in reduced lifetime of the motor, especially important in larger horsepower (hp) motors. 

Although these benefits to the electrical engineer are considerable, the benefits to the mechanical engineer are even greater. This is because the sudden impact at startup of uncontrolled starting, followed by the rapid acceleration to full speed, causes many problems across a wider range of equipment types. Which problems?

On more comprehensive units, the start voltage is pre-setable, typically from 10% to 70% of full line voltage. This should be set to achieve at least breakaway torque for the motor at start. There is little advantage in the motor sitting, staining to start due to insufficient torque. This will only increase the heat dissipated in the motor.

The start voltage setting is often referred to as the start torque setting and calibrated in percent. This is a nonsense, as although increasing the start voltage is going to increase the starting torque of the connected motor, the actual starting torque is a function of both the start voltage and the motor design.
 

Open Loop soft starters are soft starters producing a start voltage profile, which is independent of the current drawn, or the speed of the motor. The start voltage profile is programmed to follow a predetermined contour against time.

An example of open loop control is the voltage ramp. The voltage ramps from an initial voltage to full voltage in a linear fashion without regards to the load. A very basic Timed Voltage Ramp (TVR) system operates by applying an initial voltage to the motor, and causing this voltage to slowly ramp up to full voltage. On the basic systems, the initial start voltage is not adjustable, but the ramp time is. Generally speaking, the voltage ramps time is referred to as the acceleration ramp time and is calibrated in seconds.